The invention relates to a method for the manufacture of objects from polycarbonate by, inter alia, injection molding and/or compression molding. The method is suitable for the manufacture of carriers for optically readable information, such as audio and video discs.
One of the publications in which such a method has been described is Kunststoffe 77 (1987) 1, pp. 21-26, which indicates how carriers for optically readable information, such as compact discs, can be manufactured from polycarbonate granulate.
An optically readable information carrier may consist of a plastic disc, made from polycarbonate, one of whose parallel surfaces is provided with optically readable information in the form of a surface structure track of pits. The dimensions of these pits as well as the distances between them usually are of the order of magnitude of 1 to 2 micrometers. The optically readable information is covered by an internally reflecting mirroring layer, for instance an aluminum or silver layer, such that a laser beam entering the information carrier through the other surface reaches the structure track, after travelling a certain path, depending on the carrier's thickness, after which the laser beam scans this track, and is then reflected by the mirroring layer. The reflected beam, converted into an intermittent light beam due to the presence of pits, forms a signal for further processing into images and/or sound. Suitable carriers should preferably be free of double refraction caused by molecular orientation and should be of uniform thickness in all directions.
The carrier material has a stronger tendency to cause double refraction of a light beam when its surface structure is more irregular, and such double refraction decreases the probability of the reflected beam being processed as a signal. Double refraction, the phenomenon of a light beam with a certain wavelength being split into two components, each with a different refractive index, when it enters a medium, strongly decreases the probability of the reflected beam being converted into images and/or sound. The degree to which this probability decreases is determined also by the thickness of the carrier material, or the length of the path to be traversed by the beam after refraction by the carrier material. Irregularities in the thickness created during the manufacture of the disc material also have a negative effect on the quality of the images and/or sound. The same holds true for entrapped contaminations, which also affect the reflected beam. A practical requirement that may be imposed on carriers of optically readable information is that the optical path length difference, represented by .DELTA. n.times.d, is less than 50 nanometers, preferably less than 25 nanometers, .DELTA. n being the difference in refractive index of the light components and d the thickness of the carrier, expressed in nanometers. Further requirements are that, for stiffness reasons, the polycarbonate carrier has a thickness between 1.0 and 1.3 mm. For a compact disc, the thickness preferably is between 1.05 and 1.15 mm, and for purposes of standardization, the thickness is about 1.1 mm.
For the manufacture of compact discs and video discs by, for instance, the injection molding process, use is made of polycarbonate with a relatively low molecular weight. The polycarbonate should also have a low viscosity level to ensure compact discs or videodiscs are obtained that have a low level of double refraction, so that optimum information transfer via the laser beam is guaranteed. All this can be realized only when the material temperature during manufacture is relatively high, e.g., up to 360.degree. C., and when the polycarbonate has a low molecular weight. Several patent applications describe a method of achieving the above requirements by improving the thermal stability of the polycarbonate by adding end stabilizers (see e.g. Japanese patent application JP 2064-860).
One of the main problems, in spite of the thermal stabilization, is the occurrence of small black specks in the produced compact discs, which leads to a high percentage of rejects. These black specks are caused by carbonized polycarbonate particles formed during compounding of the polycarbonate granulate from the polycarbonate powder. To prevent the occurrence of these black specks and to remove them, special stabilization systems and filters are required in the polycarbonate melt just before the extruder head. Nevertheless, the percentage of granulate and, as a result, the percentage of compact discs that are rejected, remain substantial.
Black specks are a problem not only for carriers of optically readable information. They are a general problem encountered in the manufacture of objects from polycarbonate, often causing a high percentage of rejects.